This highly accurate part of the body
is considered invasive and is commonly used during surgery or
in critical care areas.

Due to the length of the esophagus, the
placement of the sensor is critical. If it is too high in the
esophagus the reading will be affected by tracheal air.

Proper placement is in the lower third
of the esophagus which will allow the sensor to be closer to
the heart and aorta, and that it will accurately reflect the
core temperature. It also indicates changes in core temperature
significantly faster than peripheral sites.

For many years rectal temperature measurement
was considered the "Standard" especially in pediatric
patients. Rectal temperature is a good approximation
of body core temperature only if the patient is in thermal balance.
Many studies have however shown that
rectal temperatures fail to track rapid changes in body core
temperature because the rectum has no thermoreceptors. In fact, because of the delayed response, core temperature
may be changing in the opposite direction, and the lag time
may be up to one hour. It is documented that
heat passes from the rectum into the blood, not vise versa.
Dwell time is significant here also. The temperature probe require
a three to five minutes dwell time. It is discussed otr possible causes of inaccurate rectal
readings, which include:

These two sites are popular with the lay
public due to their non-invasiveness and accessibility, but
the clinical accuracy at these sites is suspect. The temperature
probe must remain in position between 8 and 10 minutes. These sites are not located near a major artery or
thermoreceptor and may not reflect temperature fluctuations.
These factors may alter readings as much as 1,2° C ~ 2,2°
F lower than actual core temperature. Lastly, if the patient is in shock the peripheral
vaso-constriction will adversely affect the reading.

The most commonly used site is the sublingual
area. It is considered a fairly accurate site due to its close
proximity to the lingual and external carotid arteries, however, on average it runs lower than core temperature
by approximately 0.5C°
~ 0.8° F. Correct placement of the oral probe
is important for accuracy.

Differences in readings can vary by as
much as 0.95°
C ~ 1.7°
F from the rear sublingual pocket to beneath
the frenum in front of the floor of the mouth. Dwell time is also important, a probe require a three
to five minute dwell time. Readings
can be affected if:

The patient ate, drank, chewed gum, or
smoked within 15 minutes of the reading.

If the probe is not kept the properly
placed under the tongue.

The patient is an oral breather.

The patient talks during the reading.

It is documented that changes in oral
temperature reflect changes in blood flow not necessarily changes
in core temperature.It is furthermore stated that because of variable conditions oral temperature
should not be considered equivalent to core temperature, unless
studies are performed under strict controls.

Studies have shown a strong correlation
between bladder and other core temperatures, because the urine
is a filtrate of the blood and the kidney’s receive 20% of cardiac
output. This method is considered minimally
invasive, as it requires a urinary catheter with a thermistor
tip to be inserted into the bladder. It is explained that bladder temperatures track core temperature
changes better than rectal, but he readings may be altered due
to urinary volume or if the patient is receiving bladder irrigations.

The difference between core
(Tc) and skin temperature (Tsk), interacts physically with total
skin blood flow. The greater the (Tc - Tsk) difference, the
greater the amount of heat transferred to the body surface.
To a first approximation, the heat transfer is proportional
to the product of the temperature difference and the blood flow.

A change in either Tc or Tsk
affects heat transfer through the effect on the (Tc - Tsk) difference
and also skin blood flow, through the thermo regulatory reflex
influence of both Tc and Tsk. A Tsk change that reduced the
temperature difference (a change in the direction of reducing
heat transfer) would simultaneously induce an increase in skin
blood flow (a change in the direction of increasing heat transfer.
Arrows in the diagram show effects of an increase in skin temperature.

The first line is the physical
effect. Increased skin temperature decreases the difference
between core and skin temperatures. Consequently, less heat
is delivered to the skin surface by a given amount of skin blood
flow.

The bottom line is the reflex
effect. Increased skin temperature increases skin blood flow
with the consequence of increased heat transfer. The two effects
could balance. If the gradient was reduced by, say, 30% percent
and the increase in blood flow was also 30%, the net effect
of the skin temperature change on heat transfer could be zero.
Core temperature would remain steady despite the changes in
skin temperature and skin blood flow. If the physical and reflex
effects did balance one another, skin temperature could be driven
from one extreme of the neutral zone to the other with no effect
on core temperature. The whole job of regulation of thermal
balance would be accomplished through this reflex control from
skin temperature that perfectly balanced the physical effect
of the altered gradient.

If the effects do not balance,
then core temperature will seek a new equilibrium after a skin
temperature change. What you expect intuitively is that core
temperature should follow skin temperature. Don't you expect
that, if skin temperature fell from the high side of the neutral
range to the low extreme, core temperature would fall at least
a little? It would not if the reflex reduction of skin blood
flow were proportionately greater than the increase in core:skin
gradient. Core temperature would change the wrong direction
-- it would increase in response to a decrease in skin temperature.

As a matter of fact, that happens
all the time. Afterdrop is a familiar phenomenon in which core
temperature falls when skin temperature increases. You hear
less often of afterrise, but many have observed that a sudden
fall in skin temperature causes a rise in core temperature.
These observations are usually at skin temperatures far outside
the neutral zone, but the same phenomenon can be seen within
that narrow range.

.

As discussed earlier there are several
sites besides the inaccessible hypothalamus for measuring temperature.
How do you choose which site to use? The issues of accessibility
and accuracy are the critical thinking points of contention.
You would not put in a central monitoring urinary bladder catheter
probe just to measure temperature. Other sites are affected
by treatments and environmental factors, for example the mouth
and forehead are affected by ambient air and oral medications
or topical lotions. The rectum is affected by stool content,
and disease states such as hemorrhoids or colitis.

Obtaining a patient’s temperature by tympanic
temperature probes became available in the 1960’s. Originally,
this was done by anesthesiologists during surgery with a probe
placed directly against the membrane.

The tympanic site was chosen because it
is located in close proximity to the internal carotid artery,
which supplies the hypothalamus.
Since the tympanic membrane and auditory canal are relatively
devoid of metabolic activity, the primary determinant of the
temperature is that of the carotid artery. Tympanic temperatures are more effective in tracking
changes
in temperatures than rectal which can lag up to 60 minutes or
more behind, which in particular is important for discovering
an initial MH. The temperature of the tympanic membrane is relatively
protected from the influence of ambient temperatures and is
unaffected by smoking, respirations, eating or drinking.

MH is a life threatening, acute
pharmacogenetic disorder, developping during or after a general
anaesthesia. Both a genetic predisposition, and one or more
triggering agents are necessary to evoke MH. Triggering agents
include all volatile anaesthetics (Chloroform, Ether, Halothane,
Enflurane, Isoflurane, Sevoflurane, Deflurane) and depolarizing
muscle relaxants (Suxamethonium). The classical MH crisis shows
a hypermetabolic state, caused primarly by the muscles of the
sceletal system. Besides this classical form of MH exist abortive
forms with unspecific signs like tachycardia, arrhythmia and
a raise in temperature. Modern monitoring, better knowledge
of MH by the anaesthetists and the therapy using dantrolene
reduces the incidence of the classic MH crisis. Nevertheless,
MH is a dangerous disease, and anyone who is involved with anaesthesia
and anaesthetics should have up to date knowledge about diagnosis
and therapy of MH.